The addition of a solvent or nanoparticles to a polymeric material can lead toappreciable changes in the rates of relaxation and reaction that can profoundly alter material properties and function. We introduce a general theoretical strategy for quantifying changes in the fast molecular dynamics of mixtures based on classical transition state theory, in conjunction with arguments regarding the dependence of the entropy S+ and enthalpy E+ of activation of the fast process relaxation time on diluent concentration, xw. Based on this framework, we deduce a general classification scheme for diluents based on a consideration of the sign of the differential change in S+ and E+ with the variation of xw. Two classes of diluents exhibit a transition from plasticization to antiplasticization (speeding up or slowing down the relaxation relative to the pure system, respectively) upon varying temperature through an antiplasticization temperature , Tanti. We apply this framework to describe changes in a fast dielectric relaxation process of polycarbonate (PC) diluted by Aroclor. Many aspects of our scheme for quantifying changes in the fast dynamics of mixture are validated and, in particular, we demonstrate that the dilution of PC by Aroclor leads to a transition from antiplasticization to plasticization with decreasing temperature, in contrast to most fluids where a transition from plasticization to antiplasticization occurs upon cooling. We compare our findings from dielectric measurements with those from elastic incoherent neutron scattering and dynamical-mechanical measurements, which provide further evidence for the antiplasticization to plasticization transition phenomena observed dielectrically.
Citation: Journal of Physical Chemistry B
Pub Type: Journals
antiplasticization, dielectric relaxation, entropy-enthalpy compensation, molecular dynamics of mixtures, plasticization